1. Field of the Invention
This invention relates generally to a light-filtering arrangement for holographic displays and, more particularly, to a light-filtering arrangement for a holographic display for substantially reducing extraneous background light which would otherwise be transmitted by the hologram without substantial diffraction or absorption so as to reduce the contrast of the reconstructed stereoscopic image.
2. Description of the Prior Art
Holography, which relates to the wavefront reconstruction process by which unusually realistic three-dimensional images are reconstructed from holograms, was first discovered in 1947 by Dennis Gabor of the Imperial College of Science and Technology in London. In general, the making of a hologram requires the use of coherent light, that is, light whose waves are all in phase. The lack of an adequate source of coherent light hampered early efforts at holography. The invention of the laser in 1960 provided a source of coherent light which opened the way to new advances in wavefront reconstruction photography.
One widely used method for making a hologram requires that monochromatic, coherent light from a common source be divided into two components by such well known means as a beam splitter. One component of the coherent source light is then utilized to illuminate a subject whose photographic image is to be recorded. The light waves thereafter reflected from the subject contain all possible optical information regarding the subject and subsequently impinge upon a light sensitive or photographic plate which is exposed by the light waves reflected by the subject. The second component of coherent light is directed to impinge on the same photographic plate and has a wavefront of known and reproducable form. Since the second light component has a known and reproducable wavefront, it operates as a reference beam against which the first light wave, modulated in phase and form by the shape and optical properties of the subject, can be compared. The wavefront of the reference beam thus interferes with a reflected wavefront from the subject at the light sensitive plate to create a diffraction pattern uniquely related to the subject. A hologram is the term commonly associated with a diffraction pattern recorded in this manner. Holography differs from ordinary photography in that an enormous amount of phase modulation information is discarded through ordinary photography whereas holography utilizes optical interference with a reference beam as a means for converting phase modulation into a recordable quantity.
Viewing may be accomplished by illuminating the holographic image with a beam of monochromatic coherent light similar to that of the reference light. Light incident to the hologram at angles within a prescribed range is diffracted by the hologram. This diffraction light forms a wavefront identical in form and direction to the one originally emanating from the subject, and an observer viewing this diffracted wavefront sees a three-dimensional stereoscopic virtual image of the original subject. Viewing is generally done by looking through the plate on which the holographic image is recorded and the virtual stereoscopic image appears in relation to the hologram in a position corresponding directly with the original subject position. Thus, holography is a means of reducing three-dimensional imagry to a two-dimensional format and subsequently reconstructing the three-dimensional imagry.
More recently, techniques for producing so called information limited or low spatial frequency holograms of solid objects have been used. Such techniques are particularly advantageous in that a low spatial frequency hologram with its attendent low dispersion permits the use of ordinary broad band light for reconstruction. Thus the laser as a source of monochromatic coherent light for displaying holographic images is no longer required.
When illuminating a hologram for display purposes, only that portion of the source light incident at a predetermined illumination angle upon the hologram, such as 45.degree. above horizontal, is actually diffracted by the hologram as a result of Bragg selection effects. Light incident at other angles, such as nearly normal to the hologram plate, is transmitted therethrough without substantial diffraction or absorption, thereby degrading the reconstructed holographic images which must be viewed against a background of extraneous light reflected from objects behind the hologram. This extraneous background light which is substantially transmitted through the hologram to the eye of the viewer reduces the contrast, visibility, and apparent sharpness of the reconstructed stereoscopic image, thus necessitating that holograms be generally viewed against darkened wall and/or dimly lit rooms.
Therefore, it is a primary object of this invention to provide a holographic arrangement from which a stereoscopic image of improved contrast, visibility and apparent sharpness may be reconstructed.
It is another object of this invention to provide a holographic arrangement which may be satisfactorily displayed against a background of extraneous light emanating from behind the hologram.
It is a further object of this invention to provide a holographic arrangement which may be utilized to suitably display a stereoscopic image against a background medium other than a blackened wall.
It is an even further object of this invention to provide a holographic arrangement which may be utilized to satisfactorily display a stereoscopic image in a well lighted room.
It is an additional object of this invention to provide a holographic arrangement which accomodates the diffraction of light incident on the hologram at a prescribed illumination angle while at the same time precluding the transmission of light incident at angles other than the prescribed angle.